1. Field of the Invention
The present invention relates to a method for producing an oxide type ceramic sintered body, and more particularly to a production of a dense oxide type ceramic sintered body.
2. Description of the Related Art
With recent advancement of high density integration techniques for electronic components, development of a technique for applying fine wiring on electronic components or wiring substrates has been in progress. In many cases, these electronic components and the wiring substrates are made of oxide type ceramic sintered bodies. Generally, oxide type ceramic sintered bodies are polycrystals, and therefore pores having a size of about several tens of xcexcm, formed during a sintering process, are present at the surfaces of sintered bodies. These pores cause the problems that fine wires on electronic components or wiring substrates are interrupted, and so forth.
Known are a method of producing a hard disk substrate material as described in Laid-open Japanese Patent Application No. 5-325180, a method of producing an ITO (indium-tin oxide) sintered body as described in Laid-open Japanese Patent Application No. 6-183732, and a method of producing a piezoelectric ceramic as described in Japanese Patent Publication No. 55-19073 have been proposed.
According to the method described in the ""180 Laid-open, firing at a low temperature in the atmosphere or in a reducing or inert gas atmosphere is carried out, and thereafter, HIP (hot isotropic pressing) treatment is conducted. According to the ""732 Laid-open, proposed is a technique by which sintering at atmospheric pressure is enabled by adjustment of the grain size and compounding of raw material powders. In addition, according to the ""073 publication, grain growth can be inhibited by accelerating the cooling rate (1200xc2x0 C./hour) after firing.
The oxide type ceramic sintered body, which is an object of the present invention, is intended to be used specifically as a material for filters and wiring substrates operable in a microwave band or a millimeter wave band or as an optical material. Because of this, the materials and uses which are objects of the above-described conventional technical methods are different from those alit in the specific embodiments of the present invention.
As regards to the method described in the ""180 Laid-open, there are the problems that equipment for the HIP treatment is expensive, a reducing or inert gas atmosphere when employed as the firing atmosphere is unsuitable for firing of oxide type ceramics, the method of producing a sintered body is complicated and troublesome, and so forth.
Referring to the technique described in the Laid-open ""732, the kind of raw materials has a limitation that it is restricted to ITO. It is not necessarily possible to use another material.
Referring to the technique described in the ""073 publication, the cooling is conducted rapidly. This causes the problem that a large sintered body may be cracked.
Accordingly, it is an object of the present invention to provide a method of producing a dense oxide type ceramic sintered body of which the number of pores is extremely decreased with the above-described problems being solved.
To solve the above-described technical problems, according to the present invention, there is provided a method of producing an oxide type ceramic sintered body which comprises the steps of molding a raw material powder having a mean grain size of about 1 xcexcm or less and a BET (Brunauer, Emmett and Teller) specific surface area of about 5 m2/g or more to produce a ceramic molded body, and firing the ceramic molded body in an atmosphere of about 95% or higher oxygen concentration at a temperature higher than the lowest sintering temperature of the raw material powder for a time shorter than the time in which the sintering can be achieved at the lowest sintering temperature.
As described above, the mean grain size of the raw material powder is preferably about 1 xcexcm or less, and the BET specific surface area is also preferably about 5 m2/g or more. When the mean grain size exceeds about 1 xcexcm or the BET specific surface area is less than about 5 m2/g, pores formed in a produced ceramic molded body are excessively large. As a result, the pore size in the obtained ceramic sintered body is too large.
The raw material powder may be produced simply by mixing the oxides or carbonates of the respective metal elements to be contained in a desired oxide type ceramic sintered body, or may be powders produced by calcining the above-described mixed powders. Preferably, the purities of these raw material powders are as high as possible.
In the step of producing the above-described ceramic molded body, the ceramic molded body is produced so as to have good qualities. More concretely, when wet-molding is applied, slurry is produced so as to have a high density and a high dispersion degree. On the other hand, dry-molding is applied, granulation is carried out so as to produce grains having high crushing properties.
In the ceramic molded body obtained in the molding step, it is preferable that the pore size is small, and pores need to be evenly dispersed and small, as though the density of the molded body is low. Preferably, the maximum pore size in the ceramic molded body is up to about 0.5 xcexcm.
As described above, it is preferable that the maximum pore size in the ceramic molded body is about 0.5 xcexcm or less. This is because when pores having a pore size of more than about 0.5 xcexcm are present in the ceramic molded body, the sintering becomes uneven, which may cause the abnormal grain growth. As a result, large pores may be formed in the produced ceramic sintered body.
The firing step is carried out in an atmosphere of about 95% or higher oxygen concentration. Preferably, the oxygen concentration is as high as possible. More preferably, it is set to be about 98% or higher.
Referring to the firing temperature in the firing step and the time in which the firing step is carried out, pore growth is inhibited by firing at a lower temperature for a longer time according to conventional knowledge. However, according to the present invention, characteristically, the sintering is carried out at a firing temperature higher than the lowest sintering temperature of the raw material powder and for a time shorter than the time in which the sintering can be achieved at the lowest sintering temperature. In other words, the sintering is carried out above the temperature applied in the ordinary firing process for a time shorter than the time in which the ordinary firing process is carried out.
Referring to the firing temperature and the firing time in the above-described firing step from another standpoint, the firing time is shortened by raising the firing temperature in the firing step, whereby a tight oxide type ceramic sintered body is produced.
In addition, it is effective to set the temperature-rising rate during firing to be higher than the temperature-rising rate which is an conventional firing condition in order to obtain a tight oxide type ceramic sintered body. Preferably, a temperature-rising rate of about 180xc2x0 C./hour or higher is selected. This is because when the temperature-rising rate is less than about 180xc2x0 C./hour, the sintering becomes uneven, which may cause abnormal grain growth. Especially when a volatile component such as Pb or the like is present, the evaporation amount of the component is increased during firing. This may cause abnormal grain growth.
The present invention is applied especially advantageously when the oxide type ceramic sintered body is a dielectric ceramic sintered body.
Other features and advantages of the present invention will become apparent from the following description of the invention which refers to the accompanying drawings.